The red-siskin (Spinus cucullata) is an increasingly rare species, due to its illegal hunting, which classifies it in the status of endangered. Paramyxoviruses have been isolated from many avian species around the world, causing high morbidity and mortality in breeding grounds, commercial farms and Ecological Parks. Paramyxoviruses that infect birds belong to the Paramyxoviridae family with 21 serotypes (APMV-1-21). Serotype 2 affects Passeriformes and Psittaciformes, being more frequent in Passeriformes, causing weakness, weight loss, tracheitis, diarrhea, pneumonia and death. Passerines and Psittaciformes infected by serotype 3 may presented conjunctivitis, pancreatitis, dysphagia, dyspnea, vomiting, diarrhea, steatorrhea, in addition to neurological symptoms. Mycoplasmas are small prokaryotes belonging to the Mycoplasmatacea family. In passerines, the disease is characterized by coughing, sneezing, rales, nasal and ocular discharge and conjunctivitis. In April 2017, 2 red-siskin (Spinus cucullata) were sent to the Electron Microscopy Laboratory of the Biological Institute of São Paulo, SP, Brazil, for research of viral agents. After necropsy, samples of organ fragments (lung, heart, ventricle, liver, intestine) and feces were processed for transmission electron microscopy using the negative staining technique (rapid preparation). In the transmission electron microscope, paramyxovirus particles, pleomorphic, enveloped, containing nucleocapsid in the form of a "fishbone", measuring between 100 and 300 nm in diameter, were observed in all examined samples. In the lung fragments of the 2 birds, the presence of mycoplasma-like pleomorphic formations was also visualized, measuring between 100 and 800 nm. This report constitutes the first occurrence of these agents in Spinus cucullata.